Transition phenomena in a straight channel with a 40 to 1 aspect ratio with and without imposed pulsations, part I: near-wall and central region profiles

Abstract

A channel with a rectangular cross-section, 40 to 1 aspect ratio (height is 0.01237 m), and 4.27 m test section (336 channel height) is used to study the effects of imposed pulsations on transitional flow phenomena. Periodic velocity variations are produced in the test section using a single rotating vane located in the flow downstream of the test section. Flows with Reynolds numbers (Re) ranging from 1100 to 3600 are studies at Stokes numbers of 4.08 and 5.79 and Strouhal numbers from 0.02 to 0.122 by imposing pulsations at 1 Hz and at 2 Hz/ Time-averaged velocity profiles both within and outside of the Stokes layer are unaffected by the imposed pulsations at frequencies of 1 and 2 Hz for the entire range of Reynolds numbers studies. Longitudinal turbulence intensity profiles show a local maxima only at the channel centerline for Reynolds numbers from 1250 to 1550. This is evidence of a center node of instability. Longitudinal turbulence intensity profiles show a local maxima at both the channel centerline and y/d of 0.85 for Reynolds number from 1710 to 2300. As the Reynolds number increases from 2350 to 3400 local maxima of longitudinal turbulence intensity occur only at y/d of 0.9. Normalized profiles of longitudinal turbulence intensity are the same with and without pulsations for all Reynolds numbers studies with the exception of Reynolds numbers 2400 and 2450. Longitudinal turbulence intensity profiles at a Reynolds number of 2400 show that the magnitudes near the channel centerline are reduced at Stokes number of 5/79 (2 Hz pulsations) as compared to without pulsations. At Reynolds number 2450 the magnitude of longitudinal turbulence intensity near the channel centerline are reduced at Stokes numbers of 4/08 and 5/79 (1 Hz and 2 Hz pulsations).